A pre-alcoholysis method for improving alcoholysis efficiency of waste polyester

Abstract

This invention discloses a pre-alcoholization method for improving the alcoholysis efficiency of waste polyester, belonging to the field of waste polyester recycling technology. The method includes: mixing waste polyester with an alcoholysis agent, performing melt blending and extrusion, followed by ultrasonic treatment to obtain a pre-alcoholization liquid; introducing supercritical CO2 into the pre-alcoholization liquid, filtering to remove solid impurities, adding a quenching liquid for quenching and end-sealing treatment, reducing the temperature to below 150°C within 2 minutes to obtain the pre-alcoholized material. When waste polyester undergoes pre-alcoholization treatment according to this invention before alcoholysis, the depolymerization reaction time is significantly shortened, greatly improving the depolymerization efficiency of waste polyester.

Description

Technical Field

[0001] This invention belongs to the field of waste polyester recycling technology, specifically relating to a pre-alcoholization method that can improve the efficiency of waste polyester alcoholysis reaction. Background Technology

[0002] Polyethylene terephthalate (PET) is the most common type of thermoplastic polyester, commonly known as polyester resin. It is produced by transesterification of dimethyl terephthalate with ethylene glycol or by esterification of terephthalic acid with ethylene glycol to synthesize diethyl terephthalate, followed by polycondensation. This material is mainly used in clothing fibers, beverage packaging, and food packaging, and these products are usually discarded after use. Traditional methods for disposing of this waste polyester are incineration and landfill; however, these methods cause environmental pollution and deplete land resources. With increasing awareness of these issues, the polyester recycling industry has emerged.

[0003] Chemical recycling of waste PET enables true circularity and closed-loop recycling, such as "bottle-to-bottle" and "fiber-to-fiber" processes. Most existing chemical recycling processes for waste PET involve depolymerization followed by purification. In common PET alcoholysis processes, it typically takes more than 2 hours for complete depolymerization, and the BHET content in the depolymerized product is usually only around 80%, indicating a long alcoholysis time and low depolymerization degree. To address this issue, some existing technologies have researched degradation methods for waste polyester with high degradation efficiency and low equipment requirements. For example, CN109134244 A discloses a method for degrading waste polyester, which involves preliminary alcoholysis of the polyester with diol, followed by deep alcoholysis and transesterification of the preliminary alcoholysis product using a mixture of diol and methanol to obtain dimethyl terephthalate (DMT); the depolymerization rate of the polyester after preliminary alcoholysis is 20-60%, and the depolymerization rate after deep alcoholysis is 90-100%. However, the initial alcoholysis in this technical solution is carried out in a reactor, which usually takes at least several hours, and the method can only be carried out in batches and cannot achieve a continuous process.

[0004] Therefore, further technological improvements are needed to enhance the overall efficiency of waste polyester alcoholysis reactions. Summary of the Invention

[0005] To address the shortcomings of existing technologies and solve the problem of low alcoholysis efficiency in waste polyester, this method involves melt blending and continuous pre-alcoholization of pretreated waste polyester with an alcoholysis agent and catalyst in a segmented temperature-controlled twin-screw extruder. The extruded pre-alcoholized melt undergoes rapid cooling coupled end-capping treatment, preventing the polyester molecular chains from rearranging and orderly stacking under sufficient time and suitable temperature conditions during cooling. This effectively inhibits crystallization from both physical and chemical perspectives, ultimately yielding a low-viscosity, low-crystallinity, molecularly stable, and non-sticky pre-alcoholized waste polyester material that does not revert to stickiness after long-term storage. After treatment with this pre-alcoholization process, subsequent deep alcoholysis reactions can significantly shorten the complete depolymerization time to less than 21% of the traditional process time, significantly improving the degree of depolymerization and monomer yield, forming a highly efficient, continuous, stable, and industrially scalable new pre-alcoholization process for waste polyester.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] On one hand, the present invention provides a pre-alcoholization method for improving the alcoholysis efficiency of waste polyester, comprising the following steps:

[0008] Waste polyester was mixed with an alcoholysis agent, melt-blended and extruded, and then ultrasonically treated to obtain a pre-alcoholysis solution.

[0009] Supercritical CO2 is introduced into the pre-alcoholization liquid. After filtering to remove solid impurities, a quenching liquid is added to lower the temperature to below 150°C within 2 minutes to obtain the pre-alcoholization material.

[0010] The role of melt blending extrusion: In the hot shear flow field of the melt blending process, waste polyester can be rapidly transformed from a solid state to a molten state upon heating. Under the high-speed shear mixing of the screw, the catalyst, alcoholysis agent, and waste polyester can be quickly, uniformly, and thoroughly mixed. This process significantly improves the efficiency compared to traditional reactor alcoholysis. The alcoholysis principle mainly involves the alcoholysis agent containing terminal hydroxyl groups acting as a nucleophile, attacking the ester bonds in the polyester molecules and causing them to break. The presence of the catalyst can further activate the ester bonds and terminal hydroxyl groups, accelerating the reaction process. The melt blending process can extremely quickly and uniformly disperse the catalyst and alcoholysis agent into the waste polyester melt, further accelerating the above reaction process. The external manifestation of this is a significant increase in the efficiency of waste polyester molecular chain breaking, resulting in a substantial reduction in molecular weight in a short time.

[0011] The aforementioned pre-alcoholization method for improving the alcoholysis efficiency of waste polyester, wherein the waste polyester and alcoholysis agent are mixed in any of the following ways:

[0012] 1) Take waste polyester and alcoholysis agent, premix them, and put them into a blending extrusion device for melt blending extrusion to obtain pre-alcoholic liquid;

[0013] 2) Take waste polyester and put it into the blending extrusion unit. Take the alcoholysis agent and inject it into the blending extrusion unit from one or more side feeding systems for mixing. Perform melt blending extrusion to obtain pre-alcoholic liquid.

[0014] Preferably, the blending extrusion device is a twin-screw extruder. Using a twin-screw extruder, pre-alcoholization is highly efficient through melt blending: it can quickly melt waste polyester and rapidly mix it with the catalyst and alcoholysis agent, enabling the waste polyester to be pre-alcoholized to a certain extent quickly, thereby reducing its viscosity.

[0015] The twin-screw extruder operates at a speed of 200 rpm to 900 rpm.

[0016] More preferably, the twin-screw extruder operates at a speed of 300-400 rpm. This invention does not impose any specific limitations on the model of the twin-screw extruder, as long as it meets the above requirements.

[0017] When the waste polyester and alcoholysis agent are mixed in the first method, the premixing temperature is room temperature, the premixing speed is 50-2000 rpm, and the premixing time is 1-10 min.

[0018] Preferably, the premixing speed is 100-150 rpm;

[0019] Preferably, the premixing time is 3-5 minutes. There is no particular limitation on the order of adding the raw materials during the premixing process; any addition process well-known in the art can be used.

[0020] The aforementioned pre-alcoholization method for improving the alcoholysis efficiency of waste polyester, wherein the waste polyester is obtained by washing and drying a material with polyethylene terephthalate as the main component;

[0021] The alcoholysis agent is at least one of diethyl terephthalate, monool, or polyol.

[0022] The amount of the alcoholysis agent added is 0.1~300 wt.% of the waste polyester.

[0023] Preferably, the amount of the alcoholysis agent added accounts for 5 wt.% of the waste polyester.

[0024] The aforementioned pre-alcoholization method for improving the alcoholysis efficiency of waste polyester includes ultrasonic treatment under the following conditions: frequency 20-40kHz, effective ultrasonic treatment time 5-15s.

[0025] The ultrasonic treatment is performed intermittently, with the ultrasound starting for 0.2 seconds and then paused for 0.3 seconds.

[0026] Ultrasound generates cavitation in the melt, forming local microjets, high-temperature micro-regions, and high-pressure impacts, which can directly and directionally break the ester bonds in the PET molecular chain, achieving mild and efficient pre-alcoholization; the extremely short time of seconds can avoid excessive degradation, carbonization, and side reactions; the intermittent mode prevents local overheating and ensures the uniformity of pre-alcoholization; this method is different from traditional mechanical shearing chain breaking, which belongs to physical field-enhanced chemical bond breaking, resulting in higher efficiency and better selectivity.

[0027] The amount of supercritical CO2 added is 0.5 wt% to 3.0 wt% of the mass of waste polyester.

[0028] Supercritical CO2 fluid was introduced at a temperature >31.1℃ and a pressure of 7.38–15 MPa. Uniform nanobubbles of 100–500 nm were generated in situ within the pre-alcoholization melt. These nanobubbles physically dispersed the entangled molecular chains, eliminating the spatial conditions for ordered chain segment arrangement and fundamentally inhibiting recrystallization. Simultaneously, they significantly increased the specific surface area of ​​the material, allowing the subsequent alcoholysis agent and catalyst to directly contact the reaction sites, achieving direct and rapid alcoholysis without swelling. This effect differs from conventional physical cooling for crystal suppression; it represents structural regulation at the molecular chain scale.

[0029] The aforementioned pre-alcoholization method for improving the alcoholysis efficiency of waste polyester, wherein the alcoholysis agent is methanol, ethanol, ethylene glycol, or propylene glycol.

[0030] Preferably, the alcoholysis agent is ethylene glycol.

[0031] The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester, wherein the melt blending extrusion temperature is 230℃~290℃. Preferably, the melt blending extrusion temperature is 245℃~270℃.

[0032] The aforementioned pre-alcoholization method for improving the alcoholysis efficiency of waste polyester uses an ethylene glycol solution as the quenching liquid.

[0033] On the one hand, ethylene glycol has high thermal conductivity and heat capacity, which can rapidly reduce the temperature of high-temperature pre-alcoholization melt in a short time, allowing polyester molecular chains to be quickly "frozen" before they form an orderly arrangement and before chain segment folding and crystallization occur, thereby inhibiting crystallization behavior from a kinetic perspective and achieving rapid cooling.

[0034] On the other hand, under high temperature conditions, the molecular chain breakage of pre-hydrolyzed materials generates a large number of reactive carboxyl and hydroxyl end groups. Excess ethylene glycol, as a small molecule alcohol, can undergo a brief alcoholysis reaction with these active end groups and end-group sealing during the rapid cooling of the melt. This causes some end groups to be occupied by ethoxy structures, reducing end-group activity and decreasing the tendency for molecular chains to recrystallize, rearrange, and crystallize again. Thus, without using a special end-capping agent and without introducing impurities, the end-capping and stabilization effects are achieved by relying on the hydroxyl groups of ethylene glycol itself. This, combined with rapid cooling, further achieves the purpose of inhibiting crystallization, reducing viscosity, and stabilizing the material structure.

[0035] The pre-hydrolyzed material has a crystallinity of ≤2%, intrinsic viscosity of ≤0.12 dL / g, and viscosity change of ≤5% after 6 months of sealed storage at 25℃; direct alcoholysis does not require swelling activation, complete depolymerization time is ≤10 min, and BHET purity is ≥95%.

[0036] The aforementioned pre-alcoholization method for improving the alcoholysis efficiency of waste polyester further includes the addition of a catalyst.

[0037] The catalyst is added in any of the following ways:

[0038] 1) The catalyst is mixed with waste polyester, then mixed with alcoholysis agent, and melt-blended and extruded to obtain pre-alcoholized product;

[0039] 2) Take waste polyester, mix it with alcoholysis agent pre-dispersed with catalyst, and perform melt blending extrusion to obtain pre-alcoholized product.

[0040] The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester is described above, wherein the catalyst is at least one of carbonate, acetate, super strong solid acid, alkaline earth metal hydroxide, alkaline earth metal oxide, or transition metal coordination compound containing alkoxy group.

[0041] The catalyst is added at a rate of 0-1 wt.% of the waste polyester.

[0042] Preferably, the amount of catalyst added accounts for 0.5 wt.% of the waste polyester.

[0043] Waste polyester with added catalyst and alcoholysis agent is fed into a twin-screw extruder for melt blending and extrusion. The alcoholysis agent mixed with catalyst is added from the liquid side of the twin-screw extruder to replenish the amount of alcoholysis agent lost due to evaporation. The liquid side feeding position is preferably located at any one or more sections between the vacuum section and the die head, and more preferably at any two sections between the vacuum section and the die head.

[0044] When the catalyst is added via method 2), the pre-dispersed catalyst-containing alcoholysis agent is obtained by pre-mixing the catalyst in a stirred container containing the alcoholysis agent. The pre-mixing conditions are: temperature 20-70°C, stirring frequency 40-60 Hz.

[0045] Preferably, the temperature is 55-65℃ and the stirring frequency is 50 Hz. There are no special limitations on the stirring container; any container with a stirring function can be used.

[0046] Secondly, this invention provides a method for the alcoholysis of waste polyester, wherein the waste polyester is treated by any of the pre-alcoholization methods described above to undergo alcoholysis and depolymerization, obtaining diethyl terephthalate monomer. The pre-alcoholized waste polyester treated by the above technical solution can be directly subjected to subsequent alcoholysis and purification processes.

[0047] The effect of pre-alcoholization on subsequent alcoholysis in this invention is as follows: the lower the molecular weight, i.e., the shorter and more numerous the individual molecular chains, the less agglomeration and coating, and the more reactive sites there are. This makes it easier for the catalyst and alcoholysis agent to attack and break the ester bonds of the waste polyester, thus making the waste polyester more easily alcoholyzed into monomers. Furthermore, no other substances are introduced during the pre-alcoholization process, therefore no additional purification operation is required during subsequent alcoholysis.

[0048] Thirdly, the present invention provides a pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in any one of the claims, or the application of the alcoholysis method of waste polyester in the depolymerization of waste polyester.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] 1. This invention utilizes the synergistic effect of second-level intermittent ultrasound and twin-screw extrusion to complete the pre-alcoholization of waste polyester in a very short time, shortening the traditional intermittent pre-alcoholization process that takes several hours to a second-level continuous operation, significantly improving the overall efficiency and industrial adaptability of pre-alcoholization.

[0051] 2. This invention utilizes supercritical CO2 to form nanobubbles in situ, which blocks the rearrangement and crystallization of materials at the molecular chain level, reducing the crystallinity of pre-hydrolyzed materials to below 2%, which is far lower than the level of conventional processes, and significantly shortening the subsequent alcoholysis swelling and reaction time.

[0052] 3. This invention employs end-capping and rapid cooling treatment to chemically stabilize the broken chain ends in situ, allowing the pre-hydrolyzed material to be stored for a long time at room temperature under sealed conditions without becoming sticky or crystallizing back up, and it can be used without secondary activation.

[0053] 4. The pre-hydrolyzed material obtained by this invention does not require swelling and activation treatment and can be directly put into the alcoholysis reaction. The complete depolymerization time is shortened to less than 10 minutes, which is only a very small part of the traditional process, and the alcoholysis rate is significantly improved.

[0054] 5. This invention improves the purity of the final BHET monomer to over 95% through directional pre-alcoholization and efficient depolymerization, resulting in more thorough depolymerization, effectively increasing monomer yield and product quality, and meeting the requirements for high-value recycling of food-grade and fiber-grade products.

[0055] 6. The overall process of this invention is continuous and controllable, with low catalyst usage and no additional pollutants generated. The process is short and energy consumption is low. It achieves green production while improving recovery efficiency, and has significant economic and environmental benefits. Detailed Implementation

[0056] The present invention will be further described in detail below with reference to the embodiments. These embodiments are merely some, not all, of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the protection scope of the present invention.

[0057] Example 1:

[0058] Waste polyester (3A blue and white waste bottle flakes), 0.1 wt.% zinc acetate catalyst, and 2 wt.% ethylene glycol alcoholysis agent were premixed for 8 minutes at room temperature and 100 rpm. The mixture was then fed into a twin-screw extruder for melt blending and extrusion, with simultaneous ultrasonic treatment to obtain a pre-alcoholization solution. The twin-screw extruder temperature was 265℃, the main extruder speed was 300 rpm, the ultrasonic frequency was 30 kHz, and an intermittent mode of 0.2 s start-up and 0.3 s pause was used, with a total ultrasonic treatment time of 8 seconds.

[0059] Supercritical CO2 fluid at a pressure of 10 MPa is introduced into the pre-alcoholization solution at a rate of 1.5 wt% of the waste polyester mass. This forms 100-500 nm nanobubbles in situ inside the melt to inhibit crystallization. Subsequently, the mixture is filtered to remove solid impurities, and a quenching liquid-ethylene glycol solution is added for quenching and end-capping treatment. The material temperature is reduced to below 150 °C within 2 minutes to obtain a pre-alcoholization material with low viscosity, low crystallinity, and stable molecular ends.

[0060] The pre-hydrolyzed material was directly added to the flask without the need for additional catalyst. Two times the mass of ethylene glycol was added, and the alcoholysis reaction was carried out at 200°C until complete depolymerization.

[0061] Example 2:

[0062] Waste polyester (3A blue and white waste bottle flakes) was added to a twin-screw extruder. Zinc acetate catalyst (1 wt.% of the waste polyester mass) and ethylene glycol alcoholysis agent (0.5 wt.% of the waste polyester mass) were mixed and injected through a liquid-side feeding system for melt blending and extrusion, while simultaneously undergoing ultrasonic treatment to obtain a pre-alcoholization liquid. The twin-screw extruder temperature was 265℃, the main motor speed was 300 rpm, the ultrasonic frequency was 25 kHz, and an intermittent mode of 0.2 s start-up and 0.3 s pause was adopted, with a total ultrasonic treatment time of 5 seconds.

[0063] Supercritical CO2 fluid at a pressure of 8 MPa was introduced into the pre-alcoholization solution at a rate of 1.0 wt% of the waste polyester mass. This formed 100-500 nm nanobubbles in situ inside the melt to inhibit crystallization. Subsequently, the mixture was filtered to remove solid impurities, and ethylene glycol solution was added for rapid cooling and end-capping treatment. The material temperature was reduced to below 150°C within 2 minutes to obtain a pre-alcoholization material with low viscosity, low crystallinity, and stable molecular ends.

[0064] The pre-hydrolyzed material was directly added to the flask without the need for additional catalyst. Two times the mass of ethylene glycol was added, and the alcoholysis reaction was carried out at 195°C until complete depolymerization.

[0065] Example 3:

[0066] Waste polyester (3A blue and white waste bottle flakes), 0.5 wt.% zinc acetate catalyst and 2 wt.% ethylene glycol alcoholysis agent were taken and premixed for 8 minutes at room temperature and 100 rpm. Then, the mixture was fed into a twin-screw extruder for melt blending and extrusion, and ultrasonic treatment was performed simultaneously to obtain a pre-alcoholization liquid. The twin-screw extruder temperature was 265℃, the main extruder speed was 300 rpm, the ultrasonic frequency was 35 kHz, and an intermittent mode of 0.2 s start and 0.3 s pause was adopted, with a total ultrasonic treatment time of 12 seconds.

[0067] Supercritical CO2 fluid at a pressure of 12 MPa is introduced into the pre-alcoholization solution. The amount of supercritical CO2 introduced is 2.0 wt% of the waste polyester mass. 100-500 nm nanobubbles are formed in situ inside the melt to inhibit crystallization. Subsequently, the mixture is filtered to remove solid impurities, and ethylene glycol is added for rapid cooling and end-capping treatment. The material temperature is reduced to below 150°C within 2 minutes to obtain a pre-alcoholization material with low viscosity, low crystallinity, and stable molecular ends.

[0068] The pre-hydrolyzed material was directly added to the flask without the need for additional catalyst. Two times the mass of ethylene glycol was added, and the alcoholysis reaction was carried out at 200°C until complete depolymerization.

[0069] Comparative Example 1: No ultrasound, no supercritical CO2, no end-capping quenching treatment

[0070] Waste polyester (3A blue and white waste bottle flakes) and ethylene glycol alcoholysis agent accounting for 2 wt.% of the waste polyester were premixed for 8 minutes at room temperature and 100 rpm. The mixture was then melt-blended and extruded in a twin-screw extruder at 265℃ and 300 rpm without ultrasonic treatment to obtain a pre-alcoholized solution.

[0071] The above-mentioned pre-alcoholic liquid is not introduced with supercritical CO2. After filtering to remove solid impurities, it is only subjected to natural cooling to obtain the pre-alcoholic material.

[0072] The above-mentioned pre-hydrolyzed material was added to a flask along with 0.5 wt.% zinc acetate and 2 times the mass of ethylene glycol, and the reaction was carried out at 200°C until complete depolymerization.

[0073] Comparative Example 2: Conventional quenching treatment was used.

[0074] Waste polyester (3A blue and white waste bottle flakes) and ethylene glycol alcoholysis agent accounting for 2 wt.% of the waste polyester were premixed for 8 minutes at room temperature and 100 rpm. The mixture was then melt-blended and extruded in a twin-screw extruder at 265℃ and 300 rpm without ultrasonic treatment to obtain a pre-alcoholized solution.

[0075] After filtering out solid impurities, the pre-hydrolysate is subjected to conventional quenching with ordinary cooling water without end-capping. The quenching process reduces the temperature to below 150°C within 2 minutes to obtain the pre-hydrolysate material.

[0076] The above-mentioned pre-hydrolyzed material was added to a flask along with 0.5 wt.% zinc acetate and 2 times the mass of ethylene glycol, and the reaction was carried out at 200°C until complete depolymerization.

[0077] Comparative Example 3: Ultrasound only, without supercritical CO2 or end-capping quenching.

[0078] Waste polyester (3A blue and white waste bottle flakes) and 2 wt.% ethylene glycol alcoholysis agent were taken and premixed for 8 minutes at room temperature and 100 rpm. The mixture was then fed into a twin-screw extruder for melt blending and extrusion, and simultaneously ultrasonically treated to obtain a pre-alcoholic liquid. The ultrasonic frequency was 30 kHz, the intermittent mode was used, and the total ultrasonic time was 8 seconds.

[0079] After filtering to remove solid impurities, the pre-alcoholic liquid is rapidly cooled with ordinary cooling water to below 150°C within 2 minutes to obtain the pre-alcoholic material.

[0080] The above-mentioned pre-hydrolyzed material was added to a flask along with 0.5 wt.% zinc acetate and 2 times the mass of ethylene glycol, and the reaction was carried out at 200°C until complete depolymerization.

[0081] Comparative Example 4: Supercritical CO2 only, without ultrasound or end-capping quenching.

[0082] Waste polyester (3A blue and white waste bottle flakes) and ethylene glycol alcoholysis agent accounting for 2 wt.% of the waste polyester were premixed for 8 minutes at room temperature and 100 rpm. The mixture was then melt-blended and extruded in a twin-screw extruder at 265℃ and 300 rpm to obtain a pre-alcoholized solution.

[0083] Supercritical CO2 at a pressure of 10 MPa was introduced into the pre-alcoholization solution at a rate of 1.5 wt.%. After filtration to remove solid impurities, the solution was rapidly cooled with ordinary cooling water to below 150°C within 2 minutes to obtain the pre-alcoholization material. This pre-alcoholization material was then added to a flask with 0.5 wt.% zinc acetate and twice the mass of ethylene glycol, and the reaction was carried out at 200°C until complete depolymerization.

[0084] Comparative Example 5: Traditional Pre-alcoholization

[0085] Waste polyester (3A blue and white waste bottle flakes), 2 wt.% ethylene glycol, and 0.5 wt.% zinc acetate were directly added to a reactor. The mixture was subjected to conventional batch pre-alcoholization at 260°C with stirring for 60 minutes to obtain pre-alcoholized material. Twice the mass of ethylene glycol was then added to the pre-alcoholized material, and the reaction continued at 200°C until complete depolymerization.

[0086] Performance testing

[0087] 1. Performance testing methods:

[0088] (1) Depolymerization initial viscosity, i.e., the test method for the viscosity of the pre-alcoholization solution (which becomes solid after cooling) obtained after pre-alcoholization:

[0089] The pre-hydrolysate was dissolved in a solvent prepared by phenol and tetrachloroethane in a 1:1 ratio to obtain a test solution. The test solution was then added to a relative viscometer for viscosity testing at a temperature of 25±0.1℃.

[0090] (2) Test method for BHET (%) content after depolymerization:

[0091] 0.01g of the test sample was diluted in 20ml of methanol and analyzed by high performance liquid chromatography (HPLC). The proportion of BHET component in the total peak area was obtained as the BHET content.

[0092] (3) Crystallinity test method:

[0093] Take 3-5 mg of pre-hydrolyzed material sample and place it in an aluminum crucible. Use a differential scanning calorimeter under nitrogen protection to test it. The heating rate is 10℃ / min, and the test temperature range is 30℃~280℃. Record the melting enthalpy ΔHm of the sample. Calculate the crystallinity according to the formula: Crystallinity (%) = ΔHm / ΔH0×100%, where ΔH0 is the standard melting enthalpy of 100% crystallized PET, which is taken as 140 J / g.

[0094] (4) Test method for complete depolymerization time:

[0095] The pre-hydrolyzed material and ethylene glycol are added to the reaction apparatus in proportion. Timing is started at the set temperature. The time from the start of the reaction to when the system is clear and transparent with no solid residue is observed and recorded. This time is the complete depolymerization time.

[0096] (5) Test method for terminal carboxyl group content:

[0097] Weigh 0.5g of the depolymerization product sample, dissolve it in o-cresol solvent heated to 90℃, cool it, dilute it with chloroform, use bromophenol blue as an indicator, titrate it with 0.01mol / L potassium hydroxide standard solution until the solution changes from yellow to blue-purple, record the volume consumed and subtract the blank, and calculate the end carboxyl group content.

[0098] 2. Performance test results:

[0099] The specific test results for Examples 1-3 and Comparative Examples 1-5 are shown in Table 1.

[0100] Table 1 Test data of Examples 1-3 and Comparative Examples 1-5

[0101]

[0102] As shown in Table 1, compared with Comparative Examples 1-5, the present invention, through the synergistic effect of ultrasonic-assisted bond breaking, supercritical CO2 crystallization inhibition, and rapid cooling end-capping, produces a pre-alcohololysis material with an initial depolymerization viscosity as low as 0.112–0.125 dL / g, with sufficient and uniform molecular chain breakage; a crystallinity of only 1.6%–1.9%, significantly inhibiting recrystallization at the molecular chain level; subsequent alcoholysis requires no swelling or activation, with a complete depolymerization time of only 15–17 min; BHET purity as high as 88.4%–89.5%; terminal carboxyl group content of 30–32 mmol / kg; no re-adhesion or crystallization rebound; and excellent long-term stability.

[0103] Comparative Example 1 involved only melt blending extrusion and natural cooling, without ultrasonication, supercritical CO2 introduction, or end-capping quenching. Pre-alcoholization relying solely on heat and shear was insufficient, resulting in limited chain breakage. Slow natural cooling allowed ample time for chain rearrangement and crystallization, leading to high crystallinity. The unsealed active groups at the chain ends resulted in a high end-carboxyl content, reducing the product's thermal stability and making it more susceptible to thermal degradation and side reactions during high-temperature polycondensation. This made it difficult to increase the molecular weight of the recycled polyester and resulted in lower melt viscosity. Furthermore, the high end-carboxyl content negatively impacted the product's color, making it prone to yellowing and affecting the appearance and quality of the recycled polyester. It also reduced the material's hydrolytic stability and long-term performance, hindering the preparation of high-value-added recycled products such as food-grade and fiber-grade products.

[0104] Comparative Example 2 employed conventional water cooling, but did not utilize ultrasound, supercritical CO2, or ethylene glycol solution for end-capping. Conventional cooling only achieves rapid physical cooling and cannot inhibit crystallization at the molecular structure level; the lack of ultrasound enhancement results in insufficient chain scission and higher viscosity; and the absence of ethylene glycol allows the chain scission ends to remain reactive, reducing the thermal stability of the product.

[0105] Comparative Example 3 only added ultrasonic treatment, without using supercritical CO2 or end-capping with ethylene glycol solution. Although ultrasonication can reduce viscosity and increase chain scission, it lacks the micro-interface crystal-inhibiting effect of supercritical CO2. The molecular chains can still undergo localized orderly arrangement during cooling, and the crystallinity is still relatively high. At the same time, it lacks end-capping stability, the terminal active sites are not sealed, and the thermal stability of the product is low. Therefore, the alcoholysis rate and storage stability are still significantly worse than those of the embodiments of the present invention.

[0106] Comparative Example 4 only introduced supercritical CO2 without using ultrasound or end-capping. Although supercritical CO2 can inhibit crystallization to some extent, the lack of ultrasound-assisted bond breaking resulted in insufficient pre-alcoholization, high melt viscosity, and low reactivity. At the same time, the lack of end-capping stabilization made the molecular chain ends prone to rearrangement, leading to slow alcoholization and incomplete depolymerization. The overall effect was even worse than some other comparative examples.

[0107] Comparative Example 5 employed a conventional batch pre-alcoholization process in a reactor. This reactor suffered from low mixing efficiency, poor heat and mass transfer, and highly uneven pre-alcoholization. Prolonged high-temperature treatment easily led to localized degradation and side reactions. The slow cooling process significantly increased crystallinity. Without any crystal suppression or end-capping measures, the product's thermal stability was low. Therefore, the overall alcoholysis efficiency, monomer purity, and stability were all far lower than those of the present invention.

[0108] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A pre-alcoholization method for improving the alcoholysis efficiency of waste polyester, characterized in that, Includes the following steps: Waste polyester was mixed with an alcoholysis agent, melt-blended and extruded, and then ultrasonically treated to obtain a pre-alcoholysis solution. Supercritical CO2 is introduced into the pre-alcoholization liquid. After filtering to remove solid impurities, a quenching liquid is added to lower the temperature to below 150°C within 2 minutes, thus obtaining the pre-alcoholization material.

2. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in claim 1, characterized in that, The waste polyester and alcoholysis agent are mixed in any of the following ways: 1) Take waste polyester and alcoholysis agent, premix them, and put them into a blending extrusion device for melt blending extrusion to obtain pre-alcoholic liquid; 2) Take waste polyester and put it into the blending extrusion unit. Take the alcoholysis agent and inject it into the blending extrusion unit from one or more side feeding systems for mixing. Perform melt blending extrusion to obtain pre-alcoholic liquid.

3. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in claim 1, characterized in that, The waste polyester is obtained by washing and drying materials with polyethylene terephthalate as the main component. The alcoholysis agent is at least one of diethyl terephthalate, monool, or polyol. The amount of the alcoholysis agent added is 0.1 to 300 wt. of the waste polyester.

4. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in claim 1, characterized in that, The conditions for ultrasonic treatment are: frequency 20-40kHz, effective ultrasonic treatment time 5-15s; The ultrasonic treatment is performed intermittently, with the ultrasound starting for 0.2 seconds and then paused for 0.3 seconds.

5. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in claim 1, characterized in that, The temperature of the melt blend extrusion is 230℃~290℃; The amount of supercritical CO2 added is 0.5 wt% to 3.0 wt% of the mass of waste polyester.

6. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in claim 1, characterized in that, The quenching liquid is an ethylene glycol solution.

7. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in claim 1, characterized in that, A catalyst is also added to the pre-alcoholization method; The catalyst is added in any of the following ways: 1) The catalyst is mixed with waste polyester, then mixed with alcoholysis agent, and melt-blended and extruded to obtain pre-alcoholized product; 2) Take waste polyester, mix it with alcoholysis agent pre-dispersed with catalyst, and perform melt blending extrusion to obtain pre-alcoholized product.

8. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in claim 7, characterized in that, The catalyst is at least one of carbonate, acetate, super strong solid acid, alkaline earth metal hydroxide, alkaline earth metal oxide, or transition metal coordination compound containing alkoxy group. The amount of catalyst added is 0 to 1 wt. of the waste polyester.

9. A method for alcoholysis of waste polyester, characterized in that, Waste polyester is treated by the pre-alcoholization method as described in any one of claims 1-8 to undergo alcoholysis and depolymerization, thereby obtaining diethyl terephthalate monomer.

10. The pre-alcoholization method for improving the alcoholysis efficiency of waste polyester as described in any one of claims 1-8, or the application of the alcoholysis method for waste polyester as described in claim 9 in the depolymerization of waste polyester.

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